The present invention relates to a piezoelectric thin film element including a piezoelectric thin film having a specific shape for use in a micro pump, an ink jet recording head, a micro mirror device, a micro mechanical device, a micro machine, an ultrasonic vibrator or the like. Furthermore, the present invention relates to a process for producing the piezoelectric thin film element including the piezoelectric thin film having the specific shape by the use of a photosensitive composition capable of forming a ferroelectric thin film. Moreover, the present invention relates to an ink jet recording head having the piezoelectric thin film element.
The present invention relates to a piezoelectric thin film element of a bending mode called a bimorph or a unimorph.
In a piezoelectric thin film element of a bending mode, a lower electrode, a piezoelectric thin film and an upper electrode are laminated in this order on a substrate. Although the piezoelectric thin film is to be expanded or contracted in a direction parallel to the substrate with the application of a voltage, it is restricted by the substrate on a side of the lower electrode, to be thus flexed in a direction perpendicular to the substrate in the same manner as a bimetal.
Such a piezoelectric thin film element of a bending mode is applied to various kinds of products. The piezoelectric thin film element according to the present invention is an element suitable for, in particular, an ink jet recording head.
The ink jet recording head generally comprises ink chambers, nozzles communicating with the ink chambers and ink supplying means communicating with the ink chambers, wherein a capacity of the ink chamber is varied by the use of a piezoelectric element so that ink is discharged from the nozzle under pressure generated at that time.
In order to efficiently produce a fine piezoelectric thin film element, a piezoelectric thin film for a piezoelectric thin film element has been recently formed by applying piezoelectric paste including a starting material of ferroelectric oxide onto a substrate by a thick film method such as a screen printing, followed by firing, as disclosed in, for example, Japanese Laid-open Patent Publication No. 7-60960/1995. A method typified by the screen printing is advantageous in very efficient production of the piezoelectric thin film, but at the same time, involves some deficiencies.
One of the deficiencies is that there easily occurs variations in shape of an obtained piezoelectric thin film. FIG. 6 is a sectional view schematically showing a typical piezoelectric thin film element formed by a screen printing. In FIG. 6, a lower electrode (12), a piezoelectric thin film (13) and an upper electrode (14) are laminated in this order on a substrate (15). The upper surface of the piezoelectric thin film (13) is curved, and therefore, the bottom surface of the thin film (13) is not parallel to the upper surface thereof. Furthermore, the piezoelectric thin film (13) is thick at the center thereof but thin at the periphery thereof. The cross-sectional shape of the piezoelectric thin film (13) is ark-shaped, and therefore, each of the angles defined between a lower side and both lateral sides is acute. The shape of this piezoelectric thin film (13) depends on various conditions of an applying process, a firing process and the like, thereby variations in the shape are liable to occur. Therefore, variations in displacement characteristics are liable to occur even in elements of the same design.
Another deficiency is non-uniformity caused by easy concentration of an electric field density at the periphery of the piezoelectric thin film where an electrode interval is narrow since an distance between the upper electrode (14) and the lower electrode (12) is not fixed in the case where a voltage is applied to the piezoelectric thin film element, in the piezoelectric thin film having the sectional shape shown in FIG. 6.
Furthermore, a region contributory to a displacement of an actuator is limited, so that the displacement cannot be increased in the piezoelectric thin film having the cross-sectional shape shown in FIG. 6. That is, a region effective in the displacement of the actuator is a portion sandwiched between the upper electrode (14) and the bottom surface of the piezoelectric thin film (13) out of the entire area of the thin film. In the element shown in FIG. 6, only a portion considerably narrower than the bottom surface of the piezoelectric thin film (13) is the region effective in the displacement. If the area of the upper electrode (14) is enlarged to widen the region effective in the displacement, short-circuiting is liable to occur between the lower electrode (12) and the upper electrode (14). In view of this, it is necessary to make the area of the upper electrode (14) narrower than that of the upper surface of the piezoelectric thin film (13) so as to allow a considerable portion having no electrode to remain at the peripheral edge portion at the upper surface of the piezoelectric thin film (13). Consequently, it is impossible to effectively use the entire region of the piezoelectric thin film, thereby making it difficult to provide sufficient element characteristics.
In this manner, in the case where the piezoelectric thin film produced by the screen printing is used as an actuator for an ink jet recording head, there arises a problem that it is impossible to provide an ink jet recording head having nozzles arrayed at a high density since ink cannot be accurately discharged caused by great variation in displacement or an insufficient displacement due to a non-uniform electric field (i.e., poor conversion efficiency), as described above.
In the meantime, FIG. 7 is a sectional view schematically showing a conventional piezoelectric thin film element which has been publicly known. In FIG. 7, a lower electrode (22), a piezoelectric thin film (23) and an upper electrode (24) are laminated in this order on a substrate (25). The cross-sectional shape of the thin film (23) is rectangular, the thickness thereof is fixed and the distance between the upper electrode (24) and the lower electrode (22) is fixed, thus obtaining a uniform electric field. In order to enlarge a region effective in displacement, it is preferable that the upper electrode (24) should have a larger area. However, like the element shown in FIG. 6, it is necessary to make the area of the upper electrode (24) narrower than that of the upper surface (i.e., the area of the bottom surface) of the piezoelectric thin film (23) so as to avoid any short-circuiting between the lower electrode (22) and the upper electrode (24). That is, it is impossible to effectively use the entire area of the bottom surface of the piezoelectric thin film. Thus, there arises a problem that it is impossible to provide an ink jet recording head having nozzles arrayed at a high density.
Accordingly, an object of the present invention is to solve the above-described problems in the prior art and to provide a piezoelectric thin film element including a piezoelectric thin film having a specific shape. Furthermore, another object of the present invention is to provide a process for producing the piezoelectric thin film element including the piezoelectric thin film having the specific shape according to the present invention by the use of a photosensitive composition capable of forming a ferroelectric thin film. Moreover, a further object of the present invention is to provide an ink jet recording head provided with the piezoelectric thin film element having the specific shape according to the present invention.
As a result of earnest study, the present inventor has accomplished a piezoelectric thin film having a specific sectional shape according to the present invention. A piezoelectric thin film element according to the present invention is featured by a greater displacement and smaller scattering in piezoelectric characteristics than a conventional piezoelectric thin film element.
Additionally, as a result of earnest study, the present inventor-has accomplished the present invention by the finding of a process for producing a piezoelectric thin film having a specific shape, in which a photosensitive composition capable of forming a ferroelectric thin film is applied onto a substrate, followed by subjecting the obtained photosensitive layer to light exposure of a predetermined pattern, development and firing, as an easier process for producing the above-described piezoelectric thin film element than the conventional screen printing.
That is, the present invention relates to a piezoelectric thin film element comprising a substrate, a lower electrode formed on said substrate, a piezoelectric thin film formed on said lower electrode and an upper electrode formed on said piezoelectric thin film,
wherein a vertical cross-sectional shape and/or a vertical longitudinal sectional shape of said piezoelectric thin film with respect to said substrate is a quadrilateral having upper and lower sides mutually facing and substantially parallel to each other and both lateral sides, and
angles xcex8l and xcex8r defined between the lower side and the respective lateral sides are:
90xc2x0 less than xcex8l less than 150xc2x0 and/or
90xc2x0 less than xcex8r less than 150xc2x0.
In the present invention, the thickness of said piezoelectric thin film is preferably 1 xcexcm to 25 xcexcm.
In the present invention, it is preferable that in the quadrilateral in the vertical section of the piezoelectric thin film, a length Lu of the upper side and a length Lb of the lower side should satisfy the relationship of Lu greater than Lb.
In the present invention, the difference between the maximum thickness and the minimum thickness of said piezoelectric thin film at arbitrary points is preferably less than 1 xcexcm.
In the present invention, said piezoelectric thin film preferably comprises mainly lead zirconate titanate.
The present invention also relates to a process for producing the above-described piezoelectric thin film element. That is, the present invention relates to a process for producing a piezoelectric thin film element comprising a substrate, a lower electrode formed on said substrate, a piezoelectric thin film formed on said lower electrode and an upper electrode formed on said piezoelectric thin film,
wherein a vertical cross-sectional shape and/or a vertical longitudinal sectional shape of said piezoelectric thin film with respect to said substrate is a quadrilateral having upper and lower sides mutually facing and substantially parallel to each other and both lateral sides, and
angles xcex8l and xcex8r defined between the lower side and the respective lateral sides are:
90xc2x0 less than xcex8l less than 150xc2x0 and/or
90xc2x0 less than xcex8r less than 150xc2x0.
which comprises:
applying a photosensitive composition capable of forming a ferroelectric thin film onto the lower electrode to form a photosensitive layer; and
subjecting the photosensitive layer to light exposure of a predetermined pattern, followed by development and firing, thereby forming the predetermined pattern.
In the producing process according to the present invention, it is preferable that in the quadrilateral in the vertical section of the piezoelectric thin film, a length Lu of the upper side and a length Lb of the lower side should satisfy the relationship of Lu greater than Lb.
In the production process of the present invention, said photosensitive composition capable of forming the ferroelectric thin film is not particularly limited, but there can be mentioned e.g. the following 2 types of photosensitive compositions:
The first is a photosensitive composition (type I) comprising at least an organic polymer (A) and a ferroelectric fine particle (Bp) having photosensitive groups on at least a part of the surface thereof.
In this case (type I), said ferroelectric fine particles (Bp) comprising mainly lead zirconate titanate are preferably used. Further, non-photosensitive, ferroelectric fine particles can also be used in combination.
Further, it is also possible to use a photosensitive composition comprising said organic polymer (A), at least a part of which is a photosensitive organic polymer (Ap), and further comprising a photosensitizer (A2) and/or a photopolymerizable monomer (A3) as necessary.
When the photosensitive organic polymer (Ap) is used, it is preferable to use a photosensitive composition comprising the photosensitive organic polymer (Ap), the photosensitizer (A2) and/or the photopolymerizable monomer (A3) as necessary, and the ferroelectric fine particle (B) [the total of said ferroelectric fine particle (Bp) and said ferroelectric fine particle having no photoreactive group] at a compounding ratio of 0.5 less than (B)/[(Ap)+(A2)+(A3)+(B)] less than 0.99 in terms of the ratio of solid contents by weight.
As the photosensitizer (A2), a compound having a high coefficient of absorption is preferably selected from azide compounds, benzophenone derivatives, benzoin ether derivatives and thioxanthone derivatives.
The second is a photosensitive composition (type II) comprising at least a precursor sol of ferroelectric oxide (Sp) containing titanium, zirconium and lead as constitutional metal elements and simultaneously having photoreactive groups, and an organic polymer (A).
In this case (type II), it is possible to use a photosensitive composition comprising said organic polymer (A), at least a part of which is the photosensitive organic polymer (Ap), and further comprising the photosensitizer (A2) and/or the photopolymerizable monomer (A3) as necessary.
When the photosensitive organic polymer (Ap) is used, it is preferable to use a photosensitive composition comprising, in terms of solid content, 1 to 100 parts by weight of the organic polymer (A) relative to 100 parts by weight of the precursor sol of ferroelectric oxide (Sp).
Further in this case (type II), a photosensitive composition comprising the ferroelectric oxide fine particle (B) can also be used. As the ferroelectric oxide fine particle (B), a fine particle (Bp) having photoreactive groups on at least a part of the surface thereof can also be used. The ferroelectric fine particle (B) comprising mainly lead zirconate titanate is preferably used.
When the ferroelectric oxide fine particle (B) is used, it is preferable to use a photosensitive composition comprising, in terms of solid content, 1 to 100 parts by weight of the organic polymer (A) and 1 to 10,000 parts by weight of the ferroelectric oxide fine particle (B) relative to 100 parts by weight of the precursor sol of ferroelectric oxide (Sp).
In the present invention, the photosensitive organic polymer (Ap) can be used as a part or the whole of said organic polymer (A), and a negative-photosensitive organic polymer can be used as the photosensitive organic polymer (Ap). Further, the fine particle (Bp) having photoreactive groups can be used as the ferroelectric fine particle (B), and the ferroelectric fine particles (Bp) having negative-working photoreactive groups can be used as said fine particle (Bp).
In the producing process according to the present invention, the lower electrode is formed on the substrate by a normal process, the piezoelectric thin film is formed on the lower electrode, and then the upper electrode is formed on the piezoelectric thin film by the normal process, thereby obtaining the piezoelectric thin film element.
Moreover, the present invention relates to an ink jet recording head having the said piezoelectric thin film element. That is, the present invention relates to an ink jet recording head comprising: at least one of said piezoelectric thin film element or said piezoelectric thin film element obtained by the above-described producing process; at least one ink chamber; and at least one ink discharging nozzle.
In said ink jet recording head, assuming that Lb1 represents a length in a width direction of a bottom surface of said piezoelectric thin film and C1 represents a length in a width direction of said ink chamber, the relationship below is preferably established: 0.50 C1xe2x89xa6Lb1xe2x89xa60.95 C1, more preferably, 0.65 C1xe2x89xa6Lb1xe2x89xa60.90 C1.
In the said ink jet recording head, assuming that Lu1 represents a length in a width direction of an upper surface of said piezoelectric thin film and C2 represents a length in a longitudinal direction of said ink chamber, the relationship below is preferably established: 5xe2x89xa6C2/Lu1xe2x89xa650, more preferably, 8xe2x89xa6C2/Lu1xe2x89xa635.
In said ink jet recording head, assuming that C1 represents a length in a width direction of said ink chamber, the relationship below is preferably established: 20 xcexcmxe2x89xa6C1xe2x89xa6500 xcexcm, more preferably, 30 xcexcmxe2x89xa6C1xe2x89xa6300 xcexcm.
In said ink jet recording head, assuming that Lb1 represents a length in a width direction of a bottom surface of said piezoelectric thin film, Eu1 represents a length in a width direction of said upper electrode and Lu1 represents a length in a width direction of an upper surface of said piezoelectric thin film, the relationship below is more preferably established: Lb1 less than Eu1 less than Lu1.
In said ink jet recording head, assuming that Lb1 represents a length in a width direction of a bottom surface of said piezoelectric thin film, Lu1 represents a length in a width direction of an upper surface of said piezoelectric thin film, C1 represents a length in a width direction of said ink chamber, C2 represents a length in a longitudinal direction of said ink chamber and Eu1 represents a length in a width direction of said upper electrode, the relationships below are more preferably established:
0.50 C1xe2x89xa6Lb1xe2x89xa60.95 C1;
5xe2x89xa6C2/Lu1xe2x89xa650; and
Lb1 less than Eu1 less than Lu1.